Recently there has been proposed a DNA vector system wherein a gene coding for the target antigen is inserted into a live adenovirus which is then formulated in an enteric coated dosage form.
However, many adenoviruses are known. The selection of a suitable virus to act as a vector for the gene, and the identification of a suitable non-essential region as a site for insertion of the gene pose a challenge. In particular, the insertion site must be non-essential for the viable replication of the virus and its effective operation in vivo. Moreover, the insertion site must be capable of accepting a considerable amount of new genetic material, whilst ensuring that the virus continues to replicate. The CAV-2 adenovirus is known to be a safe vector but its DNA comprises some 31,000 base pairs which must be searched to identify a suitable site or indeed to establish whether a suitable site exists or not.